The present invention relates to a radar antenna incorporating elements radiating a pseudo-omnidirectional pattern.
The invention is applicable on the one hand to an antenna for primary radar ensuring the side lobe blanking (S.L.B.) function for the directional pattern, which consists of covering these side lobes of the directional radiation pattern emitted by the primary source of the radar with a pseudo-omnidirectional radiation pattern, whose level is higher than that of the side lobes to be covered. The invention is also applicable to a common antenna for primary and secondary radars having an interrogation system of the IFF type and which also ensures the side lobe suppression (S.L.S.) function of the directional pattern.
An antenna for filling the primary radar function has a reflector supplied in such a way that it radiates energy for the purpose of detecting a target. When, for example, this target has a sufficiently high interference level to cover the side lobes of the directional pattern radiated by the antenna particular interest is attached to the answer of this target in the axis of the major lobe of the directional pattern by attempting to mask the interference of the target by a pseudo-omnidirectional pattern. To this end a source is placed above the antenna reflector, for example a horn, which radiates such a pattern. However, this type of source has the disadvantage of being heavy and complicated.
The term common antenna for primary and secondary radars is understood to mean a single reflector supplied so as to ensure the direction function of a primary radar and which is also able to emit an interrogation signal from said target and to receive the answer from its on-board transponder, i.e. what is called the secondary radar function.
The beam carrying the interrogation is directional, interrogating in the direction where the aircraft has been detected. However, it has been found that the responder of the interrogated aircraft could be triggered by the side lobes of the interrogation pattern, whose level may be relatively high compared with that of the major lobe. Thus, errors can occur and their consequences may be dangerous. To obviate this disadvantage this single antenna is supplemented by so-called control means incorporating radiating elements acting at the reception of the interrogation by the interrogated responder and at the reception of the answer from the latter by the receiver in question. They radiate in accordance with a quasi-omnidirectional pattern, whose level is such that it covers the side lobes of the pattern radiated by the main antenna.
By comparison in the associated circuits of the amplitude of the pulses received from the responder and the control pulses this arrangement makes it possible to determine the pulses received in answer to the interrogation by the major lobe.
The control means for realizing this control diagram must be such that the gain of the associated control channels is higher than that of the interrogation and reception channels in the angular areas comprising the side lobes of the directional interrogation pattern, but much lower in the direction of their major lobe.
In the existing constructions the control means are either physically independent of the main antenna constituted by an omnidirectional antenna placed alongside the main antenna, or are dependent, the control function being fulfilled by the secondary radar antenna supplied for a given time so as to give a radiation pattern of the difference type, whilst the pattern by which the interrogation takes place is a sum pattern.
However, and despite the precautions which have been taken, it would appear that the radiation pattern of the control means does not completely fulfill its function, either because it is not totally omnidirectional, or because certain high level side lobes of the main directional pattern are not covered and also because in certain cases the major lobe, whose level is a little low, is liable to be smothered by the omnidirectional pattern. Moreover, the control patterns can be disturbed by certain external installations such as, for example, the radomes under which the antennas are placed.